Carbon exchange rates of shoots required to utilize available acetylene reduction capacity in soybean and alfalfa root nodules

The CO₂-exchange rate required to make full use of available N₂-fixation capacity, measured as acetylene reduction, was determined in soybean and alfalfa. Carbohydrates of root systems were depleted during a 40-hour dark treatment; then plants were exposed to a 24-hour light period during which different CO₂-exchange rates were maintained with various CO₂ concentrations. In three- and four-week-old soybeans and four-week-old alfalfa plants, acetylene-reduction capacity was used fully with CO₂-exchange rates as low as 10 milligrams CO₂ per plant per hour. In six-week-old alfalfa plants, however, acetylene reduction rates increased linearly, and apparent N₂-fixation capacity was not used fully when CO₂-exchange rates were higher than 40 milligrams CO₂ per plant per hour. Under the conditions established, the energy cost of N₂ fixation, measured as Δ(respiration of roots + nodules)/Δacetylene reduction over dark-treatment values, was 0.453 milligrams CO₂ per micromole C₂H₄ for all rates of acetylene reduction and for both ages of soybean and alfalfa plants. Thus, root-plus-nodule respiration was not promoted by higher rates of apparent photosynthesis after C₂H₂-reduction capacity became saturated, and all available capacity for apparent N₂ fixation had the same energy requirement.